The present invention relates to digital images. More specifically, the present invention relates to image sharpening.
Image sharpening is performed to improve the appearance of digital images and particularly the legibility of documents. One well-known image sharpening technique is unsharp masking.
However, traditional unsharp masking enhances perceptible noise in sharpened images. For instance, the unsharp masking enhances low-amplitude noise in scanned images and captured photographs. In addition, traditional unsharp masking does not avoid overshoot at edges. Overshoot can be especially troublesome if a digital image is interpolated. The overshoot is spatially spread by the interpolation and appears as an artifact in the interpolated image.
Modifications to traditional unsharp masking have been made to reduce the perceptible noise and avoid the overshoot. However, the modifications do not address imperceptible noise, which reduces the compressibility of the sharpened images. Moreover, modified unsharp masking techniques that avoid overshoot are usually slower to perform.
Another image sharpening technique, known from morphological filtering theory, is toggle mapping. Toggle mapping is usually effective for sharpening text-based images containing edges between black text and white background. However, toggle mapping is not as effective for sharpening image regions that are not purely black and white. In such regions, the toggle mapping tends to oversharpen textures and natural (e.g., photographed) features. The toggle mapping also tends to enhance noise, although not as much as traditional unsharp masking. Moreover, the toggle mapping tends to produce jagged edges in text in low-resolution images (in low resolution images, the text looks better when smoothed by anti-aliasing).